"Combined with the human-induced depletion of groundwater sources by pumping, and the extensive pollution of rivers and lakes, mass migrations may be unavoidable."

The Christian Aid agency predicts that by 2050 global warming could displace as many as 1 billion people.

"All around the world, predictable patterns are going to result in very long-term and very immediate changes in the ability of people to earn their livelihoods," Michele Klein Solomon of the International Organization of Migration told Reuters. "It's pretty overwhelming to see what we might be facing in the next 50 years. And it's starting now."

Forced migration linked to climate change is part of a larger problem: refugees due to floods, famine, and other environmental conditions. In 2002, the United Nations estimated that there were about 24 million environmental refugees.

While concern is rising for low-lying island nations and sub-Saharan countries especially vulnerable to drought, some experts say climate-caused forced migration already has happened on a large scale in North America – in the aftermath of hurricane Katrina along the Gulf Coast.

Lester Brown of the Earth Policy Institute points out that the 2005 hurricane "forced a million people from the [city] of New Orleans and other small towns on the Mississippi and Louisiana coasts in the United States to move inland, either within states or neighboring states, such as Texas and Arkansas," according to an article in Sri Lanka's The Sunday Times.

But the largest numbers of those forced to move by climate change are likely to be in developing countries, especially those threatened by desertification, AFP reported this week after the United Nations' annual World Day to Combat Desertification on June 17.

"By 2025, Africa could lose as much as two-thirds of its arable land compared with 1990, and there could be declines of one-third in Asia and one-fifth in South America. Migration – from the Sahelian regions to the West African coast, from sub-Saharan Africa to Europe, from Mexico to the United States – will be an inevitable consequence as poor people are driven off their land."

Among the most threatened are people in Bangladesh, reports the Chicago Tribune. Writes Tribune foreign correspondent Laurie Goering:

"Bangladesh is hardly the only low-lying nation facing tough times as the world warms. But scientists say it in many ways represents climate change's 'perfect storm' of challenges because it is extremely poor, extremely populated, and extremely susceptible.... The extent of Bangladesh's coming problem is evident in Antarpara, a village stuck between the Jamuna and Bangali rivers five hours northwest of Dhaka, the capital. In it and other low-lying villages nearby, more than half of the 3,300 families have lost their land to worsening river erosion. Some have moved their homes a dozen times and are running out of places to flee."

By contrast, officials in Israel worry that climate change will mean less water for them. Warns Gidon Bromberg, director of the Israeli office of Friends of the Earth – Middle East, on the website of The Jewish Chronicle of Pittsburgh:

"There will be less water available for Israel, but there will also be less water available for Israel's neighbors, and that will make [compliance with] existing peace treaty commitments more difficult between Israel and Jordan. ... And it makes difficult future agreements to be struck between Israel and the Palestinian Authority, between Israel and Syria, and between Israel and Lebanon."

"Two decades ago, the rains in southern Sudan began to fail. ... Scientists at first considered this to be an unfortunate quirk of nature. But subsequent investigation found that it coincided with a rise in temperatures of the Indian Ocean, disrupting seasonal monsoons. This suggests that the drying of sub-Saharan Africa derives, to some degree, from man-made global warming."

"The chaos that results can be an incubator of civil strife, genocide and the growth of terrorism," the 35-page report predicted. "We will pay for this one way or another," wrote retired Marine Corps Gen. Anthony Zinni, former commander of US forces in the Middle East, Central Asia, and the Horn of Africa. "We will pay to reduce greenhouse-gas emissions today, and we'll have to take an economic hit of some kind. Or we will pay the price later in military terms, and that will involve human lives."